Aerated fiber cement building products and methods of making the same

ABSTRACT

Disclosed herein are low density fiber cement articles, such as fiber cement building panels and sheets, comprised of multiple overlaying fiber cement substrate layers having small and uniform entrained air pockets and low density fillers distributed throughout. The combination of entrained air pockets and low density fillers provide a low density fiber cement matrix with physical and mechanical properties similar to comparable low density fiber cement matrix without entrained air pockets.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present disclosure relate to light-weight,low-density fiber cement material compositions and methods ofmanufacture.

Description of the Related Art

Fiber cement based building products, such as fiber cement sheets andpanels, have been widely used in building construction. Efforts havebeen made to develop various low cost methods to reduce the density andweight of the fiber cement while maintaining desirable performancecharacteristics of the material. For example, low density additives,such as ceramic microspheres, have been incorporated in fiber cement.The additives are generally selected to reduce the density of the fibercement without impairing the performance characteristics of the finalproduct in both installation and lifetime durability and performance.

It is, however, particularly challenging to develop suitable low densityadditives for fiber cement building sheets or panels comprised ofmultiple overlaying substrate layers because of the harsh processingconditions associated with making such products. In particular, most lowdensity additives have difficulty surviving the physical and mechanicalforces imparted by the Hatschek process, which is widely used formanufacturing cellulose fibers reinforced cement sheets and panels. Thelow density additives selected would have to withstand the highpressure, forces, and temperature encountered through the Hatschekprocess.

While air entrainment is a method that can be used to reduce the densityof concrete, the technique cannot be successfully and consistentlyapplied to aeration of fiber reinforced cementitious sheets or panelsfor which predictable air void content and distribution are desired. Infact, numerous studies have documented the difficulties in predictingair void content of aerated uncured concrete when subject to forces orpressure. High pressure imparted on air pockets, bursting of voids byvacuum, and rupture of voids by impact forces are some of the mechanismsfor air void losses in pumping aerated concrete. Thus, even though theconcept of aerating concrete is known, it has not been successfullyapplied to producing low density fiber cement panels or sheets becauseof the inconsistencies in the number, distribution, and size of airvoids formed by conventional air entrainment techniques. Accordingly,there is still a need for improved aeration methods and materials formanufacturing fiber reinforced panels or sheets with consistent andevenly distributed air voids.

SUMMARY OF THE INVENTION

The formulations, materials, articles, and methods of manufacture ofthis disclosure each have several aspects, no single one of which issolely responsible for its desirable attributes.

Any terms not directly defined herein shall be understood to have all ofthe meanings commonly associated with them as understood within the art.Certain terms are discussed below, or elsewhere in the specification, toprovide additional guidance to the practitioner in describing thecompositions, methods, systems, and the like of various embodiments, andhow to make or use them. It will be appreciated that the same thing maybe said in more than one way. Consequently, alternative language andsynonyms may be used for any one or more of the terms discussed herein.No significance is to be placed upon whether or not a term is elaboratedor discussed herein. Some synonyms or substitutable methods, materialsand the like are provided. Recital of one or a few synonyms orequivalents does not exclude use of other synonyms or equivalents,unless it is explicitly stated. Use of examples in the specification,including examples of terms, is for illustrative purposes only and doesnot limit the scope and meaning of the embodiments herein.

The preferred embodiments of the present disclosure provide a buildingarticle which includes a plurality of thin overlaying fiber cementsubstrate layers. Each of the overlaying fiber cement substrate layer isbonded to an adjacent overlaying substrate layer thereby forming a fibercement matrix. The building article further includes a plurality of airvoids that are defined by air pockets entrained in the fiber cementmatrix such that the air voids are formed directly in the fiber cementmatrix with no material separating the air voids from the fiber cementmatrix. The air voids are dispersed uniformly throughout the fibercement matrix to reduce the density of the fiber cement matrix such thatthe cumulative volume of the air voids is greater than 3% of the volumeof the fiber cement matrix. In some embodiments, the air voids also havean average diameter greater than 20 microns. In one embodiment, theaverage diameter of the air voids is between 20 microns and 100 microns.In yet another embodiment, the average diameter of air voids is between20 microns and 50 microns, preferably between 20 microns and 40 microns,preferably between 30 microns and 40 microns. In another embodiment, thecumulative volume of the air voids is between 5% to 20% by volume of thefiber cement matrix. In yet another embodiment, the cumulative volume ofair voids is between 10% to 20% by volume. In yet another embodiment,the density of the fiber cement matrix is between 1.0 to 1.3 grams percubic centimeter (g/cc), or between 1.2 to 1.3 g/cc in some otherembodiments. In yet another embodiment, at least 90% of the air voidsare closed-cell. In yet another embodiment, each overlaying fiber cementsubstrate layer has a thickness of between 20 to 450 microns. In yetanother embodiment, the fiber cement substrate layers include cellulosefibers.

The preferred embodiments of the present disclosure further provide afiber cement formulation which includes a hydraulic binder, cellulosefibers, and an air entrainment agent. Preferably, the air entrainmentagent comprises 0.1%-2% by weight of the formulation, or in someimplementations, 0.3%-2% by weight. In one embodiment, the airentrainment agent comprises a vinsol resin. In another embodiment, theair entrainment agent is selected from the group consisting of sodiumvinsol resin, sodium benzene sulfonate, benzenesulfonic acid, sodiumsubstituted benzene sulfonate, and combinations thereof. In yet anotherembodiment, the air entrainment agent is selected from the groupconsisting of salts of wood resins, synthetic detergents, salts ofsulfonated lignin, salts of petroleum acids, salts of proteinaceousmaterial, fatty and resinous acid and their salts, alkylbenzenesulfonates, salts of sulfonated hydrocarbons, and combinations thereof.In yet another embodiment, the air entrainment agent is selected fromthe group consisting of wood resins, sulfonated hydrocarbons andcombinations thereof. In yet another embodiment, the air entrainmentagent comprises a sacrificial filler, such as a blowing agent.

The preferred embodiments of the present disclosure further provide amethod of entraining air in a fiber cement panel comprising multipleoverlays of substrate layers. The method includes the steps of creatingair bubbles in a aqueous fiber cement slurry, depositing the fibercement slurry as thin fiber cement films on a plurality of sievecylinders that are rotated through the fiber cement slurry. The airbubbles are distributed uniformly in the thin fiber cement films. Themethod further includes the steps of transferring a series of sequentiallayers of the thin fiber cement films to a belt so as to build a thickerfiber cement layer, removing water from the thicker fiber cement layer,and curing the thicker fiber cement layer. In one embodiment, the airbubbles are created in the aqueous fiber cement slurry by adding anaerating agent directly to the fiber cement slurry and vigorously mixingthe fiber cement slurry. In another embodiment, the air bubbles arecreated in the aqueous fiber cement slurry by vigorous premixing anaerating agent in water to create a foam mixture and then adding thefoam mixture to the fiber cement slurry.

The preferred embodiments of the present disclosure further provide afiber cement formulation comprising less than 50% by weight of asiliceous aggregate, about 25%-38% by weight of a hydraulic bindingagent; about 5%-15% by weight of cellulose fibers; about 1.5%-3.5% byweight of microspheres; about 3%-8% by weight of expanded perlite, andabout 0.1%-0.5% by weight an air entrainment agent. In one embodiment,the formulation comprises about 1.5% by weight of the microspheres,about 3.5% by weight of the expanded perlite, and about 0.5% by weightof the air entrainment agent. In another embodiment, the formulationcomprises about 3.5% by weight of the microspheres, about 3.5% by weightof the expanded perlite, and about 0.5% by weight of the air entrainmentagent.

The preferred embodiments of the present disclosure further provide abuilding article comprising a plurality of thin overlaying fiber cementsubstrate layers, wherein each of the overlaying fiber cement substratelayer is bonded to an adjacent overlaying substrate layer therebyforming a fiber cement matrix. The building article further comprises afirst plurality of filler particles having a first density andincorporated in the fiber cement matrix, wherein the first plurality offiller particles comprises about 0.1%-5% by volume of the fiber cementmatrix. The building article further comprises a second plurality offiller particles having a second density and incorporated in the fibercement matrix, wherein the second plurality of filler particles have adensity less than the density of the first plurality of fillerparticles. The building article further comprises a plurality of airvoids defined by air pockets entrained in the fiber cement matrix suchthat the air voids are formed directly in the fiber cement matrix,wherein the air voids comprise about 3%-20% by volume of the fibercement matrix, wherein the air voids are closed cell. In one embodiment,the first plurality of filler particles comprises microspheres. Inanother embodiment, the second plurality of filler particles comprisesperlite. In yet another embodiment, the air voids have an averagediameter greater than 20 microns. In yet another embodiment, the averagediameter of the air voids is between 20 microns to 100 microns.

The preferred embodiments of the present disclosure further provides afiber cement formulation comprising a binding agent which comprises5%-80% by weight of the total weight of the formulation, cellulosefibers which comprises 5%-15% by weight of the total weight of theformulation, a first filler comprising 1%-15% by weight of the totalweight of the formulation, a second filler comprising 1%-10% by weightof the total weight of the formulation, wherein the second filler has adensity less than that of the first filler, and wherein an airentrainment agent comprising 0.6%-5.0% by weight of the total weight ofthe formulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a low-density fiber cement articleaccording to one embodiment of the present disclosure.

FIG. 2 is a SEM photo of voids dispersed throughout the fiber cementarticle of FIG. 1 shown at 100× magnification.

FIG. 3 is a SEM photo of voids dispersed throughout the fiber cementarticle of FIG. 1 shown at 400× magnification.

FIG. 4 is a SEM photo of the structure of a single air void of the fibercement article of FIG. 1.

FIG. 5 shows a comparison of pore diameter distribution of low densityfiber cement product of a preferred embodiment and equivalent lowdensity fiber cement product having ceramic microspheres as additive.

FIG. 6 illustrates the results of MOR testing of an embodiment of aproduct as compared to a control product.

FIG. 7 illustrates the results of ILB testing of an embodiment of aproduct as compared to a control product.

FIGS. 8A-B show SEM photographs at 100× magnification of an embodimentof a product as compared to a control product.

FIG. 9 illustrates flexural strength (expressed as modulus of Rupture(MoR), toughness, modulus of elasticity and strain control of anembodiment of a product as compared to a control product.

FIG. 10 illustrates Archimedes Density of an embodiment of a product ascompared to a control product.

FIG. 11 is a schematic illustration of a process flow of one embodimentfor manufacturing a low-density aerated fiber cement sheet inconjunction with the Hatschek process;

FIG. 12 is a schematic illustration of a process flow of anotherembodiment for manufacturing a low-density aerated fiber cement sheet inconjunction with the Hatschek process; and

FIG. 13 are SEM photos comparing an aerated low-density fiber cementsheet of one preferred embodiment with a control low-density fibercement sheet made with microsphere additives.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Disclosed herein are low density fiber cement articles comprised ofmultiple overlaying fiber cement substrate layers having small anduniform entrained air pockets distributed throughout. Also disclosedherein are air entrainment systems and methods for manufacturing aeratedlow density fiber cement panels with consistent air void content anduniform air void distribution. Also disclosed herein are air entrainmenttechnologies adapted to work in conjunction with the Hatschek process toproduce aerated fiber cement articles having controlled air void contentand distribution.

FIG. 1 illustrates one embodiment of a low density aerated fiber cementarticle 100 of the present disclosure. The fiber cement article 100includes a plurality of overlaying fiber cement substrate layers 102,which together form a fiber cement matrix 106. As shown in FIG. 1, aplurality of air voids 104 are formed in the fiber cement matrix 106 byair entrained in the matrix 106. The air voids 104 are preferably sizedwithin a pre-selected range and are uniformly distributed throughout thefiber cement article 100. In one implementation, the diameter of the airvoids 104 is greater than 20 microns (μm), preferably between about 20μm to 100 μm. In another implementation, the average diameter of the airvoids 104 is between about 20 μm to 60 μm. In another implementation,the air voids 104 are more than 5% by volume of the fiber cement article100, preferably between about 5%-20% by volume, preferably between about5%-10% by volume, preferably between about 9.2% by volume.

FIG. 2 is a SEM photo of air voids 104 dispersed throughout the fibercement matrix 106 shown at 100× magnification. FIG. 3 is a SEM photo ofair voids 104 shown at 400× magnification. FIG. 4 is a SEM photoillustrating the structure of a single air void 104 shown at 3000×magnification. As shown in the SEM photos, the air voids 104 formed inthe fiber cement matrix 106 are generally mixtures of single andclusters of air bubbles. Unlike low-density additives such asmicrospheres, the air voids 104 are air pockets formed directly in thefiber cement matrix by air entrainment such that the walls defining eachvoid is part of the fiber cement matrix. The air voids are preferablyclosed cell voids that do not form continuous air channels with adjacentvoids. In some embodiments, at least 50%, 60%, 70%, 80%, 90%, or over90% of the air voids in the article are closed cell voids. The shape andvolume of the voids are preferably determined by the amount of airentrained in the fiber cement article. While air entraining techniqueshave been used to aerate concrete, it is very difficult to formuniformly distributed predominantly closed cell air voids in fibercement panels or sheets using conventional concrete aeration techniques.

One of the challenges in forming uniformly distributed, closed cell airvoids with the desired morphology in fiber cement panels or sheets isthe difficulty in entraining air bubbles in the fiber cement that cansurvive the Hatschek process intact. The Hatschek process is widely usedas a commercially viable method for making fiber cement panels or sheetswith multiple overlaying substrate layers. To the best of the inventors'knowledge, aeration of fiber cement building sheets produced by theHatschek process has never been considered as a viable process forforming uniform air voids because of the difficulty in producing airbubbles or pockets in the low solid content slurry and the bursting ofair bubbles due to the numerous vigorous processing steps of theHatschek process. Some of the processing steps that present obstacles touniform air bubble formation are the slurry agitation, pumping, andfiltering steps; the fiber cement film generation and fiber cement layerbuildup steps; the high vacuum treatment of the thicker fiber cementlayer and multilayer fiber cement green sheet; the high nip pressure anddensity increasing roll pressures on the green sheet; and the highpressure long autoclaving curing step.

Furthermore teachings from the aeration of concrete, which typically haswater to cement ratios of less than 1, provide no guidance indetermining whether or not the aeration of fiber cement building sheetsproduced by the Hatschek process would be feasible. For example, studieshave shown difficulties in predicting the air content of aerated uncuredconcrete when subject to forces or pressure. Some have recommendedagainst pumping air-entrained concrete at pressures in excess of 300psi. Additionally, the effect of air entrainment additives on concretecement is difficult to predict because of the numerous factors affectingperformance. For example, excessive vibration can result in as much as50% of entrained air being lost after three minutes of vibration; veryfine (<150 μm) and coarse (>1200 μm) aggregates decrease air content,but aggregates between 150-1200 μm increase air content; and an increasein concrete temperature will significantly decrease the air content.

The size of the air bubbles present in the uncured concrete cement canalso be affected by the forces imposed on it. Suction and dissolutionthat occur in pumping concrete could result in very few, if any, airbubbles with diameters below 50 μm being present afterwards. Suctionoccurs in pumping when the wet concrete is subject to a negativepressure brought on by vacuum when the pump piston chamber fills withthe wet concrete paste. Dissolution occurs when the wet concrete pasteis subject to pressure with the smaller air bubbles dissolving in waterbut not re-forming when the wet concrete paste is depressurized.Furthermore the current knowledge in the field of entrained air in freshuncured concrete paste appears to be that the solubility of theentrained air is the same as that for free air and water. As such, basedon Henry's Law, a fresh concrete paste that is subject to a pressure of400 psi will likely dissolve a total amount of air comprising 52% ofwater volume present.

Accordingly, as explained above, to the best of the inventors'knowledge, there is no information from the aeration of concrete and theproperties of fresh concrete paste that teaches that the aeration offiber cement building sheet or panel with multiple overlaying substratelayers made using a Hatschek process would be technically feasible.Given the pressures, forces and temperatures encountered by the materialprocessed through the Hatschek process, all previous low cost method ofreducing the density of fiber cement building sheets and panels havebeen based on the addition of a low density additive, such as hollowceramic or glass microspheres.

The present disclosure provides various embodiments of fiber cementarticles having multiple overlaying substrate layers with air pocketsdistributed throughout. Preferably, the fiber cement article is madeusing a Hatschek process. In some embodiments, air pockets areintroduced into the matrix by air entrainment during the manufacturingprocess of the fiber cement article. In other preferred embodiments, airpockets are introduced by the use of sacrificial materials in the fibercement formulation wherein the materials disintegrate during themanufacturing process, leaving air pockets within the matrix of the endproducts. The fiber cement article may be a panel or a sheet and may beused as a building product.

As discussed in greater detail below, air entrainment according topreferred embodiments of the disclosure may be achieved by vigorousmixing of the formulation slurry in the Hatsheck process to produce airbubbles, coupling with the use of suitable aerating agents for bubblestability. In some preferred embodiments, the mixing condition isadjusted so that air bubbles are substantially within the 20 to 100 μmin diameter. It has been found that higher mixing rates produce smallerbubble size. In some embodiments, air bubbles are pre-produced as afoam, using a high sheer foam generated pump, before being introducedinto the slurry.

Aerating agents as used herein can be chemicals or compounds that havethe ability to help bubble generation and/or bubble stabilization withinthe Hatschek process. In some embodiments, these chemicals can be longchain polymers, acting as surfactants to strengthen the bubble shells.While there are many types of long chain polymers, the selection ofsuitable aerated agents for cellulose fiber cement is not obvious. Theprocess water in Hatschek process for fiber cement product typicallycontains dissolved form of cements such as Calcium (Ca²⁺), Sodium (Na⁺),Potassium (K⁺), and Sulfate (SO₄ ⁻). Thus, the process water is high inpH and selected aerating agents must be able to survive in a high pHenvironment. In some embodiments, preferred aerating agents are cationicsurfactants made with ammonium substitute, anionic surfactant such aswood resin, fatty acid salt or synthetic detergent, with carboxylategroup (R—COO—), sulfate group (R—SO₄—) and sulfonate group (R—SO₃—)).More preferred aerated agents are sodium vinsol resin and/or sodiumbenzene sulfonate. In some embodiments, aerating agents are used in theamount of between 0.1% to 5% by weight or 0.3% to 2% by weight of thetotal fiber cement formulation. In more preferred embodiments, theamount of aerating agents used is 1% to 3% by weight. In someimplementations, aerating agents are used in conjunction with other lowdensity additives, such as microspheres, to reduce the density of fibercement article. In one implementation, the aerating agents compriseabout 0.13% by weight of the total fiber cement formulation andmicrospheres comprise about 1% by weight of the total fiber cementformulation.

Other preferred embodiments utilize suitable sacrificial materials orfillers to produce the air voids in the fiber cement matrix. Thesacrificial materials or fillers are selected for their ability todisintegrate during the Hatschek process, leaving air pockets in thematrix of the end product. In some preferred implementation, thesacrificial fillers can be starch, ammonium carbonate and/or sodiumbicarbonate by consideration of cost efficiency. Sacrificial material ispreferably introduced into the formulation in a finely ground form sothat it can be mixed and distributed throughout the formulation duringthe manufacturing process, thus leaving well distributed air pocketswithin the matrix of the end product. Preferred particle sizes of thesacrificial materials are less than 250 μm (60 mesh).

Concrete Air Entrainment Additives (AEA)

In some implementations, certain concrete air entrainment additives(AEA) are selected and modified for fiber cement sheets or panels formedby the Hatschek process. There are four commercially available types ofAEAs used in the concrete cement industry: wood resins, syntheticdetergents, petroleum acid salts, and fatty acids salts. Laboratorytests on the foam production from commercially available AEAs mixed withhighly alkaline fiber cement processing water were performed todetermine whether any of the AEAs could be used as a potential airentrainment additive for fiber cement sheets and panels. The test wasconducted using a mixer (KitchenAid 325) with 500 ml of highly alkalineprocessing water from a fiber cement Hatschek manufacturing process and4 grams of the selected AEA being mixed in a bowl at gear 8 of the mixerfor 10 minutes. The foaming ability of the AEAs was measured bymeasuring the height of the resulting foam from the base of the mixingbowl.

TABLE 1 Foam test for AEA in highly alkaline fiber cement processingwater. AEA type Commercial Name Detailed Description of AEA Foaming (mm)Wood MB-VR Sodium vinsol resin 137.2 resins Polychem VR Sodium vinsolresin 100 Super Air Plus Powder VR no foam Petroleum salts Polychem AESubstituted benzene sulfonate (Na salt) 73.75 (unstable) NASA HS 90Sodium benzene sulfonate 118.13 Micro Air Sodium olefin sulfonate nofoam Airen S Benzenesulfonic Acid (Na salt) 85 Fatty acid salts EuconAir 40 Tall oil. PCMC. Glycol ether no foam Hydrophobe Complexcarbohydrate, polyalcohols, etc. no foam Synthetic Chemical Sodiumlauryl sulfate no foam detergents

As shown in Table 1, the fatty acids salt and the synthetic detergentAEAs were deactivated in highly alkaline fiber cement process water withno air bubble formation. Sodium vinsol resin and sodium benzenesulfonate successfully produced foam from the highly alkaline fibercement processing water.

A cement hydration test was conducted using differential calorimetermethod on cement slurry having a water to slurry ratio (w/s ratio) of0.4. The test showed that sodium benzene sulfonate was a strong cementhydration retarder with a 0.5% by weight addition to cement, whichresulted in the cement not setting within 72 hours. On the other hand,sodium vinsol resin had little effect on cement hydration. Since theprocess water for fiber cement product has a high in pH due to itsconstituents, additional laboratory tests were conducted and it wasfound that sufficient and controllable levels of entrained air weredifficult to attain with most concrete AEAs.

To further assess why the fiber cement process water prevented mostconcrete AEAs from foaming, the following tests were performed usingMicro Air for sodium olefin sulfonate, which did not foam with the fibercement processing water, and sodium vinsol resin. The test andmeasurements are the same as in the fiber cement processing water testexcept the amount of AEA was reduced by 50% to 2 grams of the AEA inquestion.

TABLE 2 AEA foaming ability in different alkaline water AEA TypeSolution Foaming (mm) Micro Air Sodium olefin Fresh water (pH 7) Good(73 mm) sulfonate Micro Air Sodium olefin NaOH water (pH 13) Good (73mm) sulfonate Micro Air Sodium olefin Ca(OH)₂ water No foaming sulfonate(pH 13) (35 mm) MB-VR Sodium vinsol resin Fresh water (pH 7) Good (76mm) MB-VR Sodium vinsol resin NaOH water Good (76 mm) (pH 13) MB-VRSodium vinsol resin Ca(OH)₂ water Good (76 mm) (pH 13)

From the results of Table 2, it appears that the presence of lime in theprocess water deactivates the Micro Air AEA from foaming. Accordinglyfor an AEA to potentially work in the fiber cement articles, the AEAneeds to work in high pH, and possibly high Ca⁺ ion concentrations.

Aerated Fiber Cement Sheets Using Sodium Vinsol Resin as an Additive

Fiber cement building sheets made by the Hatschek process with theaddition of sodium vinsol resin was conducted and a separate trial withadditions of hollow microspheres for comparison. It was found that 0.5%concentration of sodium vinsol resin addition to the formulation slurryfor fiber cement produced the same fiber cement building sheet densityas a formulation using 3% of hollow microspheres. The productperformance of fiber cement building sheets using sodium vinsol resin asan additive is shown in Table 3:

TABLE 3 Fiber cement building sheet performance comparison Sodium vinsolresin as percent of total Moisture Movement solids in slurry MoR ILB(Pre-Carb/Post-carb) Density 0% (control) 10.1 1.7 0.18/0.53 1.28 0.1%9.7 1.43 0.18/0.53 1.25 0.5% 9.4 1.4 0.18/0.51 1.23

A separate study using fiber cement building sheets produced by theHatschek process and using 2% sodium vinsol resin produced an averagefiber cement product density 1.15 g/cm³. Further scanning electronmicroscope (SEM) analysis of the resulting fiber cement with aerationshows the air bubble particle size in the fiber cement building sheetsto range between 20 μm to 100 μm. In addition, SEM analysis also showsthat although the morphology of the air bubbles appear to be irregular,possibly due to the nip pressure, the entrained air bubbles remainunbroken and isolated in fiber cement sheet without the presence of airchannels, which might occur due to excessive vacuuming forces. Alsoanalysis of the fiber cement building sheets found the overall airpockets accounted for about 9.2% of the board by volume with theentrained air distributed evenly throughout.

A comparison of pore distribution in low density fiber cement productsthat was made by sodium vinsol resin and ceramic microsphere as lowdensity additive with the same density of 1.15 g/cm³ are shown in FIG.5. As shown, in some embodiments, fiber cement products incorporatingaerating agents show a percent increase in pore volume in certain poresize ranges. In one implementation, there is about a 20% volume increasein pores ranging from 0.01 to 0.1 microns in diameter. In anotherimplementation, fiber cement products incorporation aerating agents showa distribution of pore diameters from 0.01 to 0.1 micron, 1 to 10microns, and 10 to 100 microns. In yet another implementation, the fibercement products incorporating aerating agents show an average porediameter of greater than 20 microns, preferably between 20 to 100microns, constituting at least 50% of the volume of the pores and atleast 3% of the volume of the fiber cement product. Furthermore, anaddition of 0.5% sodium vinsol resin to total slurry solids by weightwas found to produce about a 4% decrease in product the post autoclavedensity of the fiber cement building sheet and about a 7% decrease insaturated MoR when compared to a control fiber cement building sheet.For the moisture expansion property, building sheets using sodium vinsolresin showed parity to the control building sheets. To understand howthe addition of sodium vinsol resin compares to spheres in fiber cementproducts made by the Hatschek process, we compared the density reductionand performance with 0.5% sodium vinsol resin sampled from this trialand microsphere addition trial. From the analysis, for density reduction0.5% of sodium vinsol resin to the total solids is equivalent to 3-4% ofmicrospheres addition to the same. In addition Modulus of Rupture (MoR)performance as measured in according to ASTM C 1185, Clause 5 in wetcondition shows parity between the two fiber cement building sheets.

Some preferred embodiments also control the size of bubble introductioninto the fiber cement slurry. Since smaller bubbles have higher surfacetension than larger bubbles, it was found that bubble size within therange of 20 to 100 microns is best to survive the Hatschek process. Theprocess can be engineered by using a high slurry agitation or highslurry pumping rate or introduction of a designated high shear pump forbubble formation.

Sacrificial Fillers

Other preferred embodiments provide a method of manufacturing aeratedfiber cement products by careful selection of suitable sacrificialfillers that will be configured to disintegrate during the Hatsheckprocess, leaving air pockets in the matrix of the end product. A rangeof potential sacrificial fillers were tested for the Hatschek fibercement process. The test results are shown in Table 4. The filtrationdensity reduction percentage was calculated by comparing the filtercakes obtained from the vacuum filtration of cement/silica slurry withand without a sacrificial filler. The pad press density reductionpercentage was calculated from fiber cement pads produced from a Wabashhydraulic press machine (model PC-75-4TM) using 28 ton/ft² compressionpressure From the results, it was found that only a few selectedmaterials are suitable for aeration purposes.

TABLE 4 Testing of Sacrificial Fillers as potential additives foraerating fiber cement building sheets Density Reduction Addition UnderUnder Pad Variable (addition) Level filtration Press Acrylic copolymer0.70% −7.2%  — encapsulating blowing agent Polyacrylamide water 0.30%−3.7%   — absorbent polymers Carbopol (swelling 0.50% −3.8%   — polymer)PVA (PVOH)   1% — — Organic Blowing Agent 0.40%  −4% — (BenzeneSulphohydrazide (BSH)) Organic Blowing Agent 0.80% −12% — (BenzeneSulphohydrazide (BSH)) Aluminum Powder 0.55% −12.5%   −4% AmmoniumNitrate 1.50%  −8.5%   — Ammonium Sulfate 1.50% −11% — NaHCO₃ 1.5%Filter cake  −9% — 2% Pad Press −3.3%   NaHCO₃   6% −32% −6% Starch 1.5%Filter cake −4.5% — 2% Pad Press −1.9%   Starch   8% −28% −8% AmmoniumBicarbonate 1.5% Filter cake −13% — 1% Pad Press −4.9%   AmmoniumBicarbonate   6% −42% −11%  Urea   2%  −7% −2%Combination of Air Entrainment and Low Density Fillers

In some embodiments, a combination of air entrainment with certain lowdensity fillers can be used to reduce the density of a fiber cementproduct formed by the Hatschek process while still maintaining adequatemechanical properties. By using a combination of air entrainment andcertain low density fillers in accordance with the embodiments disclosedherein, density reduction in the product can be achieved in a more costeffective manner than using fillers alone.

In some embodiments, a predetermined amount of one or more densityfillers or modifiers such as, for example, lightweight inorganic fillerscan be incorporated in a layered fiber cement substrate in combinationwith a predetermined amount of air bubbles formed by AEA. The airbubbles and fillers are disposed in a packing configuration within eachlayer such that the air bubbles can effectively replace fillers withoutaffecting the mechanical properties of the product. Therefore, costeffective density modification can be achieved by the simultaneousinclusion of void spaces via air bubble inclusion and lightweightinorganic fillers without sacrificing any physical and mechanicalproperties. In preferred embodiments, the inorganic fillers are selectedso that they do not react with other components in the fiber cementformulation and primarily serve the function of density modification.

Using a combination of AEA and low density fillers according to certainpreferred embodiments can lead to significant cost savings. Theinventors have found that AEA in combination with selected low densityfillers arranged in certain packing configurations within a layeredfiber cement substrate can result in substantially the same densityreduction and the same or better mechanical properties as an equivalentfiber cement product with low density fillers as the primary means fordensity reduction. In some implementations, a small amount of AEA canreplace a much larger quantity of low density fillers and still maintainsubstantially the same reduced density and mechanical properties. In oneexample, 0.5% of formulation weight addition of an AEA, when arranged ina preferred packing configuration, can replace about 50% of formulationweight of low density fillers without adversely affecting the mechanicalproperties of the final product. Thus, there can be significant costsavings in formulation per volume of product produced. Further, thedensity modification can be achieved without disrupting the formation ofmatrix rich layers in a fiber cement product manufactured using theHatschek process. Moreover, the mechanical properties and handleabilityof the product can be maintained, despite replacing a significantproportion of closed cell density filler/modifier with entrained air.Additionally, the durability properties of the product can be maintainedby modifying the internal structure of the product to include entrainedair.

In various embodiments, more than one type of low density fillers can becombined with AEA to modify the density in a fiber cement formulation.In one embodiment, the density modifying agents or low density fillersinclude a first filler, a second filler and AEA. The preferred ratio ofthe first filler to the second filler in the formulation is between 1:1and 1:2.5. Preferably, the first filler has a higher density relative tothe second filler. The first filler can have a density of between 0.5g/cm³ and 1.1 g/cm³, and comprise about 1-15 weight %±0.5 weight % ofthe total weight of the fiber cement formulation, or about 1.5-9 weight%±0.5 weight % of the total weight of the fiber cement formulation, orabout 1.5-3.5 weight %±0.5 weight % of the total weight of the fibercement formulation. The second filler can have a density of between 0.5g/cm³ and 1.2 g/cm³, and comprises about 1-10 weight %±0.5 weight % ofthe total weight of the fiber cement formulation. In someimplementations, the second filler comprises 3-4 weight %±0.5 weight %of the total weight of the fiber cement formulation. In someimplementations, the second filler can comprise about 3.50 weight %±0.5weight % of the total weight of the fiber cement formulation. The higherdensity first filler can be, for example, microspheres. The lowerdensity second filler can be, for example, an inorganic material, suchas, diatomaceous earth, slag, granulated blast furnace slag, volcanicash, expanded clays, expanded vermiculite, expanded perlite, and pumice.In this embodiment of the invention the first and second filler togetherwith AEA are sized such that each of the first and second fillers seattogether within a fiber cement product and the AEA is interspersedtherebetween. Advantageously, this configuration of the fiber cementproduct results in substantially the same density reduction and the sameor better mechanical properties when compared to an equivalent fibercement product which has a single low density filler as the primarymeans for density reduction. Furthermore, the combined percentage weightof the first and second fillers and AEA is less than the percentageweight of the single low density filler, thus substantially lowering thecost of production of the fiber cement product.

Combination of AEA and Microspheres

In some embodiments, the density modifying agents include a combinationof AEA and microspheres arranged in a certain packing configurationwithin a layered fiber cement substrate formed by the Hatschek process.The microspheres can be cenospheres, silicate spheres, borosilicatespheres, aluminosilicate spheres, and the like. The preferredmicrospheres generally comprise about 30-85% SiO₂, 2-45% Al₂O₃, up toabout 30% divalent metal oxides, and less than 10% alkali metal oxides.Such microspheres generally have a bulk density of less than about 1.4gm/cm³. In certain preferred implementations, the microspheres have adensity of between 0.6 to 0.9 g/cm³ and an average diameter of 30-500microns. In some implementations, the microspheres comprise a singletype of microsphere. In other implementations, the microspheres comprisemore than one type of microspheres that fall within the preferreddensity and size ranges. The AEA can comprise about 0.6-5 weight % ofthe total weight of the formulation. In some other implementations, theAEA can comprise about 0.45-0.55 weight % of the total weight of theformulation, or about 0.5 weight % of the total weight of theformulation.

The AEA and microspheres can be incorporated into a fiber cement slurryprepared for the Hatschek process to form layered fiber cementsubstrates. In some embodiments, the formulation of the fiber cementslurry comprises a binding agent, aggregates, fibers, and otheradditives. The binding agent can be, for example, a hydraulic bindingagent such as cement. The binding agent can comprise between about 5-80weight % of the total weight of the formulation. In some embodiments,the binding agent can comprise about 20 to 50 weight % of the totalweight of the formulation. In some embodiments, the binding agent cancomprise about 25 to 38 weight % of the total weight of the formulation.In some embodiments, the binding agent can comprise about 26 to 38weight % of the total weight of the formulation. The aggregate cancomprise, for example a siliceous source and an alumina source.Siliceous materials can include sand, quartz rock, fume silica, andmanufactured siliceous materials such as calcium silicate hydrate.Combined siliceous and alumina containing materials can includenaturally occurring alumina-silicate minerals including clays, felsicminerals, mafic minerals, and ultramafic minerals. Alumina material caninclude synthetic aluminosilicate minerals such as glass, and heattreated aluminosilicate minerals such as metakaolin. In someembodiments, the aggregate can be ground silica, amorphous silica,silica fume, diatomaceous earth, rice hull ash, blast furnace slag,granulated slag, steel slag, mineral oxides, mineral hydroxides, clays,magnasite, dolomite, metal oxides, metal hydroxides, and mixturesthereof.

The siliceous aggregate can comprise between about 10-60 weight %, or15-60 weight % of the total weight of the formulation. In someembodiments, the siliceous aggregate can comprise about 40-50 weight %of the total weight of the formulation. In some embodiments, thesiliceous aggregate can comprise about 43.7-46 weight % of the totalweight of the formulation. The alumina aggregate portion can comprisebetween about 0-5 weight % of the total weight of the formulation. Insome embodiments, the alumina aggregate can comprise about 3.4-3.5weight % of the total weight of the formulation. In some embodiments,the alumina aggregate can comprise about 3.45 weight % of the totalweight of the formulation.

In some embodiments, both alumina and siliceous aggregate are used. Insome embodiments, only one of the two is used. In some embodiments, anaggregate material can include both siliceous and alumina aggregate. Insome embodiments, the fibers comprise cellulose fibers and/orpolypropylene fibers. In some embodiments, cellulose fibers are refinedto a degree of freeness of between 20 and 800 Canadian Standard Freeness(CSF), more preferably 200 to 500 CSF. Other forms of fibers may beused, examples of which include, but are not limited to, ceramic fiber,glass fiber, mineral wool, steel fiber, and synthetic polymer fiberssuch as polyamides, polyester, polypropylene, polymethylpentene,polyacrylonitrile, polyacrylamide, viscose, nylon, PVC, PVA, rayon,glass ceramic, carbon, or any mixtures thereof. Useful reinforcingcellulose fibers may also include chemically treated cellulose fibers,such as fibers treated with hydrophobicity agents, biocides, or otherchemicals.

The fibers can comprise between about 5-15 weight % of the total weightof the formulation. In some embodiments, the fibers can comprise about7-8 weight % of the total weight of the formulation.

In some embodiments, additives can be added into the mixture. Forexample, dispersing agents, fire retardant, viscosity modifiers,thickeners, pigments, colorants, dispersants, flocculating agents,water-proofing agents, natural and synthetic minerals polymeric resinemulsions, hydrophobic agents, or mixtures thereof can be used. Watercan further be added into the mixture to form a slurry. Water can beadded in sufficient quantities to make a slurry viscosity suitable for aselected forming process.

EXAMPLES

Table 5 shows fiber cement formulations A, B and C for slurries used ina Hatschek process. Formulation A utilizes no AEA and only microspheresas modifying agents, while Formulation B utilizes a combination ofmicrospheres and AEA to reduce the density of the final product. Asshown in Table 5, in Formulations B and C, about 0.5 wt. % and 0.18 wt %of AEA is added respectively and about 3 to 4 wt % of low densityadditive filler. It is to be understood that the formulations A, B and Care provided without any tolerances and when tolerances are taken intoconsideration the formulation will sum to 100 weight %.

TABLE 5 Formulations Ingredients A B C Cement 28% 28% 28% Aggregate58-59% 61% 61% Cellulose  7%  7%  7% Microspheres 6-7%  3%  3% AEA 0 ~0.5%   ~0.18%  

FIG. 8A provides an SEM image of a fiber cement product having 6 wt. %microspheres as density modifier and no AEA. FIG. 8B provides an SEMimage of an equivalent fiber cement product having 3 wt. % microspheresand 0.5 wt. % AEA as density modifiers. The microspheres in both sampleshave diameters of approximately 150 μm. A comparison of the images showthat the sample incorporating AEA also includes air bubbles having adiameter of about (replace with 0.5-50 μm), which creates voids in thefiber cement similar to microspheres.

Further testing was done for combinations of microspheres and AEA. Table8 shows the composition of a product using different levels ofmicrospheres and AEA.

TABLE 8 Compositions showing different AEA levels 0.5% 0.13% AEA 2% AEAAEA Ctrl 1 Ctrl 2 4% Sphere 0% sphere 1% sphere Ingredients PercentagesCement 38.2% 28.4% 28.4% 38.2% 38.2% Aggregate 51.4% 54.7%   58% 53.4%52.4% to 55.6% Cellulose  7.8%  7.0%  7.0%  7.8%  7.8% Additive  0.6%Balance Balance Balance 0.47% LDA   2%   7%   4%   0%   1% MicroSphereAEA —  0.5%   2% 0.13%

Table 9 below illustrates the physical properties of a control producthaving all microspheres, a product having a combination of microspheresand AEA, and a product having only AEA.

TABLE 9 Physical Properties for Different AEA Combinations 0.5% 2% 0.13%AEA AEA AEA Ctrl Ctrl 4% 0% 1% 0.25% Test 1 2 sphere sphere sphere AEAMoR Sat 10.8 7.6 7.1 7.1 10.9 11 Strain Sat 7441 8913 8482 GS (Green1.12 1.13 1.1 Sheet) Density Oven Dried 1.27 1.26 1.27 DensityCombination of AEA and Expanded Perlite

In some embodiments, the density modifying agents include a combinationof AEA and expanded perlite arranged in a certain packing configurationwithin a layered fiber cement substrate formed by the Hatschek process.Table 11 illustrates the composition of different fillers used in thetesting boards. In the examples provided in Table 11, the formulation ofthe fibre cement article remains substantially the same with theexception of the type and % weight of the low density fillers. In thefirst and second examples, perlite is used to replace microspheres. InExample 1, the percentage of perlite in the perlite only compositioncorresponds to approximately 80% of the microsphere control dosage whilein the perlite and 0.3% AEA composition, the perlite dosage correspondsto approximately 87% of the microsphere+0.3% AEA control dosage.

In Example 2, the percentage of perlite in the perlite only controlcomposition corresponds to approximately 65% of the microsphere controldosage while in the perlite and 0.3% AEA composition, the perlite dosagecorresponds to approximately 44% of the microsphere control dosage.

TABLE 11 Compositions using Microspheres and/or Perlite Sphere or Sphereor Perlite dosage Perlite dosage (%) (%) Formulation Example 1 Example 2Microsphere Control 5.5 5.2 Perlite Only 4.4 3.4 Microsphere + 3.0 —0.3% AEA Control Perlite + 0.3% AEA 2.6 2.3

Table 12 shows the density and of boards formed using microspheres,perlite, or perlite with AEA. As shown, the use of perlite and AEA canretain significantly the same density as that of microsphere or perlitealone.

TABLE 12 Density Wet Wet Density Dry Density Density Dry DensityFormulation Example 1 Example 1 Example 2 Example 2 Microsphere Control1.60 1.12 1.61 1.13 Perlite Only 1.62 1.12 1.63 1.15 Perlite + 0.3% AEA1.63 1.14 1.63 1.16

TABLE 13 shows the post autoclave test results of mechanical propertiesfor Examples 1 and 2 of Table 11.

TABLE 13 Mechanical Properties using Perlite MOR Post Autoclave DryDensity Test Example 1 Example 2 Example 1 Example 2 Microsphere 8.56.71 1.19 1.19 Control Perlite Only 7.2 6.31 1.15 1.18 Perlite + 8.06.42 1.17 1.21 0.3% AEA

Accordingly, the combination of perlite and AEA can achieve similarmechanical properties than perlite on its own.

Combination of AEA, Expanded Perlite, and Microspheres

In some embodiments, the density modifying agents include a combinationof AEA, expanded perlite, and microspheres arranged in a certain packingconfiguration within a layered fiber cement substrate formed by theHatschek process.

Tables 14 and 15 show formulations that can be used for embodiments ofthe disclosure that include multiple fillers as well as AEA.

TABLE 14 Compositions Formulation Control Combination 1 Combination 2(A3) (D3) (D4) Component Weight % Weight % Weight % Pulp 7 7 7 Cement 3638 37 Aggregate 44 46 45 Total Additives Balance Balance BalanceMicrosphere 9 1.5 3.5 Expanded Perlite 0 3.5 3.5 AEA 0 0.50 0.50 TotalWeight 100 100 100

TABLE 15 Compositions Formulation Combination 1 Combination 2 ComponentWeight % Weight % Pulp  5-15  5-15 Cement 25-38 25-38 Aggregate Lessthan 50 44-50 Total Additives Balance Balance Microsphere 1.5-3.5 3.5Expanded Perlite 3-8 3-8 AEA 0.1-0.5 0.50 Total Weight 100 100

FIGS. 9-10 illustrate different mechanical properties of embodiments ofa product as discussed in the composition table 14 above. As shown, byincorporating perlite and AEA, the mechanical properties can remainapproximately the same as when using microspheres alone.

TABLE 15 Physical Properties for AEA Combinations from Table 14 CategoryTest Ctrl A3 D3 D4 Physical Density 1.2 1.24 1.21 PropertiesProcessing

FIG. 11 illustrates a process 600 for forming a low density, aeratedfiber cement article using the Hatschek process. The process 600 isdesigned to entrain air in a fiber cement matrix such that the air voidsformed therein are uniform, predictable and can survive the harshconditions of the Hatschek process. In this embodiment, air is entrainedin the fiber cement panel to form the air voids during the Hatschekprocess. The process 600 begins with Step 610 in which one or moreaerating agents is vigorously mixed with an aqueous slurry comprisingwater, cellulose fiber, silica, cement and additives, including lowdensity additives. In one implementation, the aqueous slurry containsabout 8 to 15% solids by weight, with the cement accounting for aboutone-third of the total solids by weight. In some embodiments, theaqueous slurry can be mixed by a shear pump at a speed of greater than450 rpm or preferably greater than 500 rpm. The water to cement ratiofor the aqueous slurry can be around 20-35. The aerating agent ispreferably vinsol resin, more preferably sodium vinsol resin, comprisingabout 0.3 to 2 wt. % of the aqueous slurry. The inventors havesurprisingly found that aerating agents, when present in this weightpercent range, provide desirable buffering and other properties in thefiber cement matrix. The aerating agent can be added directly to theaqueous slurry in this embodiment. In some implementations wheremicrospheres or other low density additives are typically used to reducedensity of the fiber cement article, aerating agents can be added, incombination with microspheres to reduce the amount of microspheresneeded, hence reducing the cost of the material. In one implementation,the aerating agents comprise about 0.1-0.2 wt %, preferably 0.13 wt. %of the fiber cement formulation and microspheres comprise about 0.1 wt.% of the fiber cement formulation.

The process 600 continues with Step 620 in which the aqueous slurrycontaining the air bubbles is delivered to a feed sump and pumped to aplurality of vats. Each vat has its own agitators to ensure a consistentslurry mixture. The fiber cement material in the aqueous slurry in thevats is then deposited as thin fiber cement films on a plurality ofsieve cylinders that are rotated through the slurry in the vats in Step630. The thin fiber cement films are then layered on top of each otherto produce a thicker fiber cement layer in Step 640. The process 600produces the thicker fiber cement layer from the thin fiber cement filmsby the use of a specialized felt-based belt that both drives therotating sieve cylinders through the slurries in the vats and picks upthe thin fiber cement films deposited on the rotating sieve cylinders.

The rotating sieve cylinders can be rotated by the bottom run of thespecialized felt-based belt and filter the slurry through a sieve screenattached to the sieve cylinder. As the slurry filters through this sievescreen a thin film layer of fiber cement material is deposited on thesurface. The thin film of fiber cement material has a thickness ofbetween 0.20 to 0.45 mm (20-450 μm) and contains between 60%-75% solids(with a water to cement ratio of 1.4 to 2.2 when cement comprises 30% ofthe solids). The excess water from the slurry then passes through thesieve screen as filtrate and exits from the end of the sieve cylinder,enabling recovery and recirculation of the slurry solids in the vat. Asthe apertures of the sieve screen are typically between 0.30 to 0.50 mm(300-500 μm) all non-fibrous material that is significantly smaller than300 μm is either washed through the sieve screen or caught by thecellulose fibers that form a mesh over sieve screen. Accordinglyentrapment of small non-fibrous materials depends on the formation of aninitial filter layer of cellulose fibers on the surface of the sievescreen.

The fiber cement film formed on the surface of the sieve screen on eachsieve cylinder is then transferred upon contact to the outer surface ofthe specialized felt-based belt. This transfer process takes placebecause the felt is less porous than the sieve screen. As thespecialized felt-based belt passes over each successive vat in theseries it picks up a corresponding series of sequential layers of fibercement films from the associated sieve cylinders and builds the thickerfiber cement layer. Thereafter the thicker fiber cement layer passesover a vacuum box positioned along the top run of the belt to remove thewater and moisture still present in Step 650. Given the thickness of thethicker fiber cement layer and the water content present the vacuumsettings can range from 18 to 25 inches of mercury.

The thicker fiber cement layer then passes between a tread roller, whichalso provides the driving force for the specialized felt-based belt, andan adjacent accumulation or “size” roller in the form of a relativelylarge diameter drum. The tread and size rollers are positioned such thatfurther water is pressed out of the thicker fiber cement layer while itis transferred to the size roller by a mechanism similar to that bywhich it was previously transferred from the sieve cylinders to thebelt. The pressure in the nip between the tread and size roller, alsoreferred to as the nip pressure, is anywhere between 350 psi to 1000 psidepending on the processing requirements. Preferably, the pressure isselected so as to not remove a minimal number of air bubbles in thefiber cement layers. The size roller accumulates a number of thickerfiber cement layers according to the number of turns allowed before thelaminated sheet is cut off. Thus, the formation of a fiber cement sheetwith multiple thin substrate layers is achieved by allowing a largernumber of turns before cutting the film. In the cut off process in Step660, a wire or blade is ejected radially outwardly from the surface ofthe size roller to cut longitudinally the cylinder of laminated fibercement material that has cumulatively formed on the surface of theroller.

Once cut, the sheet of laminated fiber cement material peels off thesize roller to be removed by a run-off conveyor. The material at thisstage has the approximate consistency of wet cardboard, and thereforereadily assumes a flat configuration on the run-off conveyor. Tocomplete the process at the wet end, the felt is cleaned as it passesthrough an array of showers and vacuum boxes, before returning to thevats to pick up fresh layers of film. It will be appreciated that thequality and characteristics of laminated sheet material produced fromthe process depends on a wide range of variables associated with theslurry formulation and the various settings at the wet end of themachine. Further down the process line this material, known as “greensheet”, is roughly trimmed to size at a green trim station using highpressure water jet cutters, after which it proceeds as individual sheetsto a stacker. At the stacker, the green sheets are picked up by vacuumpads and formed with interleaving sheets into autoclave packs. Afterpartial curing, and optionally a further compression process to increasedensity, the green sheets are loaded into an autoclave unit for finalcuring under elevated temperature and pressure conditions in Step 670.In the autoclave, a chemical reaction occurs between the raw materialsto form a calcium silicate matrix which is bonded to the cellulosereinforcing fiber. This process is undertaken in 160-190° C. atsaturated steam pressure for 8 to 12 hours. At its completion, thesheets emerge fully cured, ready for accurate final trimming, finishingand packing in Step 680. The resulting product is an aerated,low-density fiber cement panel or sheet with multiple overlayingsubstrate layers. The fiber cement panel has air bubbles evenlydistributed throughout the fiber cement matrix. The air bubbles arepreferably closed-cell and have an average diameter ranging from 10 to100 μm. In some embodiments, over 80% of the air bubbles areclosed-cell. In some other embodiments, over 90% of the air bubbles areclosed-cell.

FIG. 12 illustrates another process 700 for forming a low density,aerated fiber cement article using the Hatschek process. In process 700,instead of adding the aerating agent directly to aqueous fiber cementslurry, the aerating agent is first mixed with water, and optionallyother additives, using a high shear pump to form a foam mixture in Step710. The foam mixture is then added to and mixed with the aqueous cementslurry in Step 720. Premixing the aerating agent with water can resultin more uniformly distributed air bubbles once the foam mixture is addedto the aqueous slurry.

FIG. 13 are SEM photos comparing an aerated low-density fiber cementsheet made in accordance with the methods described herein with acontrol low-density fiber cement sheet made with microsphere additives.As shown in FIG. 13, the size of the air bubbles in the aerated fibercement sheet are controlled maintained in the range of between 10 to 150μm, which is comparable to the size of the microspheres. However, theaerated fiber cement sheet is less costly to manufacture as it does notrequire the addition of microsphere or other additives to createuniformly distributed air voids in the fiber cement matrix.

From the foregoing description, it will be appreciated that an inventiveproduct and approaches for aerated fiber cement materials are disclosed.While several components, techniques and aspects have been describedwith a certain degree of particularity, it is manifest that many changescan be made in the specific designs, constructions and methodologyherein above described without departing from the spirit and scope ofthis disclosure.

Certain features that are described in this disclosure in the context ofseparate implementations can also be implemented in combination in asingle implementation. Conversely, various features that are describedin the context of a single implementation can also be implemented inmultiple implementations separately or in any suitable subcombination.Moreover, although features may be described above as acting in certaincombinations, one or more features from a claimed combination can, insome cases, be excised from the combination, and the combination may beclaimed as any subcombination or variation of any subcombination.

Moreover, while methods may be depicted in the drawings or described inthe specification in a particular order, such methods need not beperformed in the particular order shown or in sequential order, and thatall methods need not be performed, to achieve desirable results. Othermethods that are not depicted or described can be incorporated in theexample methods and processes. For example, one or more additionalmethods can be performed before, after, simultaneously, or between anyof the described methods. Further, the methods may be rearranged orreordered in other implementations. Also, the separation of varioussystem components in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described components and systems cangenerally be integrated together in a single product or packaged intomultiple products. Additionally, other implementations are within thescope of this disclosure.

Conditional language, such as “can,” “could,” “might,” or “may,” unlessspecifically stated otherwise, or otherwise understood within thecontext as used, is generally intended to convey that certainembodiments include or do not include, certain features, elements,and/or steps. Thus, such conditional language is not generally intendedto imply that features, elements, and/or steps are in any way requiredfor one or more embodiments.

Conjunctive language such as the phrase “at least one of X, Y, and Z,”unless specifically stated otherwise, is otherwise understood with thecontext as used in general to convey that an item, term, etc. may beeither X, Y, or Z. Thus, such conjunctive language is not generallyintended to imply that certain embodiments require the presence of atleast one of X, at least one of Y, and at least one of Z.

Language of degree used herein, such as the terms “approximately,”“about,” “generally,” and “substantially” as used herein represent avalue, amount, or characteristic close to the stated value, amount, orcharacteristic that still performs a desired function or achieves adesired result. For example, the terms “approximately”, “about”,“generally,” and “substantially” may refer to an amount that is withinless than or equal to 10% of, within less than or equal to 5% of, withinless than or equal to 1% of, within less than or equal to 0.1% of, andwithin less than or equal to 0.01% of the stated amount.

Some embodiments have been described in connection with the accompanyingdrawings. The figures are drawn to scale, but such scale should not belimiting, since dimensions and proportions other than what are shown arecontemplated and are within the scope of the disclosed inventions.Distances, angles, etc. are merely illustrative and do not necessarilybear an exact relationship to actual dimensions and layout of thedevices illustrated. Components can be added, removed, and/orrearranged. Further, the disclosure herein of any particular feature,aspect, method, property, characteristic, quality, attribute, element,or the like in connection with various embodiments can be used in allother embodiments set forth herein. Additionally, it will be recognizedthat any methods described herein may be practiced using any devicesuitable for performing the recited steps.

While a number of embodiments and variations thereof have been describedin detail, other modifications and methods of using the same will beapparent to those of skill in the art. Accordingly, it should beunderstood that various applications, modifications, materials, andsubstitutions can be made of equivalents without departing from theunique and inventive disclosure herein or the scope of the claims.

What is claimed is:
 1. A building article comprising: a plurality ofthin film overlaying fiber cement substrate layers, wherein each of saidoverlaying fiber cement substrate layer is bonded to an adjacentoverlaying substrate layer, the fiber cement substrate layers togetherforming a single fiber cement matrix wherein said fiber cement matrixcomprises an air entrainment agent; a first plurality of fillerparticles having a first density and incorporated in the fiber cementmatrix, wherein the first plurality of filler particles comprises about0.1%-5% by volume of the fiber cement matrix; a second plurality offiller particles having a second density and incorporated in the fibercement matrix, wherein the second plurality of filler particles have anaverage density less than the average density of the first plurality offiller particles; and a plurality of air voids formed by the airentrainment agent defined by air pockets entrained in the fiber cementmatrix such that the air voids are formed directly in the fiber cementmatrix with no material separating the air voids from the fiber cementmatrix, wherein the air voids comprise about 3%-20% by volume of thefiber cement matrix, wherein the air voids are closed cell and have anaverage diameter greater than 20 microns, and wherein the air voids areuniformly distributed.
 2. The building article of claim 1, wherein thefirst plurality of filler particles comprise microspheres.
 3. Thebuilding article of claim 1, wherein the second plurality of fillerparticles comprise perlite.
 4. The building article of claim 1, whereinthe average diameter of the air voids is between 20 microns to 100microns.
 5. The building article of claim 4 comprising a fiber cementformulation, the fiber cement formulation comprising: less than 50% byweight of a siliceous aggregate; about 25%-38% by weight of a hydraulicbinding agent; about 5%-15% by weight of cellulose fibers; about1.5%-3.5% by weight of hollow microspheres; about 3%-8% by weight ofexpanded perlite; about 0.1%-0.5% by weight of an air entrainment agent;and a hollow microsphere to air entrainment agent ratio between about 3and
 35. 6. The fiber cement formulation of claim 1, wherein theformulation comprises about 1.5% by weight of hollow microspheres, about3.5% by weight of expanded perlite, and about 0.5% by weight of the airentrainment agent.
 7. The fiber cement formulation of claim 1, whereinthe formulation comprises about 3.5% by weight of the hollowmicrospheres, about 3.5% by weight of the expanded perlite, and about0.5% by weight of the air entrainment agent.
 8. The fiber cementformulation of claim 1, wherein the air entrainment agent is selectedfrom the group consisting of salts of wood resin, synthetic detergents,salts of sulfonated lignin, salts of petroleum acids, salts ofproteinaceous material, fatty and resinous acids and salts thereof,alkylbenzene sulfonates, salts of sulfonated hydrocarbons, andcombinations thereof.
 9. The fiber cement formulation of claim 1,wherein the air entrainment agent comprises vinsol resin.
 10. The fibercement formulation of claim 1, wherein the air entrainment agentcomprises a sacrificial filler.
 11. The fiber cement formulation ofclaim 1, wherein the air entrainment agent forms a plurality ofclosed-cell voids interspersed between the microspheres and expandedperlite.